QCM (quartz crystal microbalance) is utilized for assaying interactions between biological substances such as DNA and proteins and for assays using antigen-antibody reactions.
Assaying with QCM is carried out by putting a buffer solution in contact with a quartz crystal oscillator electrode and adding a solution containing a sample as a subject to be assayed to the buffer solution, to measure the frequency variation occurring due to the binding of the sample to a substance immobilized on the quartz crystal oscillator electrode.
In that case, for example, the sample may happen to be precipitated on the bottom of the buffer solution or the sample may happen to float on the liquid surface so that the solution cannot be mixed together. Consequently, the solution cannot reach a constant final concentration, which causes inaccurate assays of the binding amount or causes no success in the binding rate analysis.
As one method for solving the problems, there is provided for example a method comprising placing an agitation rod in a container with a buffer solution injected therein and vibrating the solution therein along the upward and downward directions mechanically or rotating the solution (for example, patent reference 1).
Since a certain amount of the solution is needed so as to enable agitation of the solution by the method, however, the solution of a trace volume cannot be agitated sufficiently, disadvantageously. Because the solution cannot be agitated sufficiently, then, it is hard to carry out assays at high precision.
Due to the vibration of the quartz crystal oscillator with an electric power applied during assaying, furthermore, waves such as share wave and compressional wave generate. In case of an AT-cut quartz crystal oscillator of 27 MHz, share wave is reduced at a distance of about 100 nm from the surface of the quartz crystal oscillator, so that such wave cannot be used for agitation. It has been revealed experimentally that compressional wave itself cannot agitate liquefied materials.    Patent reference 1: JP-A-2002-310872